BELFAST, Northern Ireland — Travel along the Irish Republic border with Northern Ireland today and you will follow a tortuous line through rolling farmland, along lakes and rivers, beginning at the edge of one ocean, the Atlantic, and ending by the shores of another, the Irish Sea. The frontier is marked by the skeletons of army watchtowers and checkpoints built at the height of Northern Ireland’s Troubles, and now largely abandoned.

In the border towns of Londonderry and Newry, and in the villages and small towns in between, the normal rhythms of life are beginning to return after three decades of sectarian violence in Northern Ireland, and deep economic decline on both sides of the frontier.

The border has its origins in the creation of the Irish Free State in 1921, after the war against colonial Britain.

A Boundary Commission was set up to decide where the frontier should be drawn between the British statelet and its newly independent southern neighbor, which went on to become the Republic of Ireland in 1948.

Northern Protestants, called unionists because of their support for remaining part of Britain, had a clear majority over Catholics in only four northern counties — Antrim, Down, Londonderry and Armagh. This was deemed too small a territory to form a viable unionist entity, so the predominantly Catholic counties of Fermanagh and Tyrone were added to the emerging British province of Northern Ireland.

Sliced in two

The arrangement contained many anomalies, not least of which was the division of Ireland’s Province of Ulster. Three of Ulster’s counties — Donegal, Cavan and Monaghan — were now in the Irish Free State. Countless farms were divided between the Irish Free State and Northern Ireland, and the city of Londonderry — now in Northern Ireland — was cut off from its natural hinterland of Donegal.

Many people were unhappy, especially Protestant unionists stranded on the Irish Republic side of the border and Catholic nationalists trapped in Northern Ireland.

The Boundary Commission surveyed residents of the border areas to gauge their wishes, but concluded in 1925 that any changes would be incompatible with economic and geographic considerations. The fate of six-county Northern Ireland was sealed.

MSNBC.com’s Laura Haydon traveled along the border recently to see how people there have fared since the line between Northern Ireland and the Irish Republic was drawn 75 years ago — and since the advent of peace and the economic prosperity of the last several years.